Methods and apparatus for coating a wide variety of workpieces by the electrostatic deposition of a particulate material thereon are well known in the art. Highly satisfactory and effective electrostatic coating apparatus is provided by fluidized bed devices in which a mass of coating powder is fluidized by passage of a gas therethrough. Generally, such equipment has a horizontally disposed plate or membrane for support of the powder, and air is charged through the plate from an underlying plenum. The particles are electrostatically charged by passage proximate to an electrode, which is usually mounted adjacent the porous plate and is connected to a high voltage D.C. source. Apparatus of this type is commercially available from Electrostatic Equipment Corporation, of New Haven, Conn.
Although such devices are in widespread use for numerous applications, their full potential has not heretofore been realized. A significant deterrant to wider utilization has been the inherent hazard presented by the high voltage charging means. Thus, the possibility of arcing occurring between the electrode and the workpiece or to personnel in the vicinity, with the consequent threats of fire and electrical shock that are presented, has not only limited acceptance, but has also frequently imposed undue design restrictions. To a lesser extent, the conventional electrode arrangement has itself imposed limitations upon design flexibility.
Attempts have been made to provide less dangerous electrostatic fluidized bed equipment, exemplary of which is the device described in Sargent U.S. Pat. No. 3,670,699, wherein a gas transmissive web of moderate conductivity is disposed in the gas path, so that the gas may be employed to carry electrical charges to the fluidized particles. Although devices of the sort described by Sargent may offer some advantages, their charging efficiency tends to be inadequate and to vary with changing ambient conditions (such as humidity) and, moreover, they are found to afford a generally inadequate measure of safety.
Accordingly, it is an object of the present invention to provide novel electrostatic fluidized bed apparatus that is highly safe to operate.
It is also an object of the invention to provide such apparatus, which is capable of efficiently, reliably and uniformly charging particulate materials.
Another object is to provide such apparatus in which a high degree of design flexibility is afforded.
A further object of the invention is to provide a novel method for electrostatic coating, which is highly safe.
A still further object is to provide a novel method whereby particulate materials can be electrostatically charged efficiently, reliably and uniformly.